1. Field of the Invention
The present invention relates to a voltage regulator that generates a constant output voltage from an input voltage.
2. Description of the Related Art
In general, an electronic device such as a mobile phone operates on a rechargeable battery. A voltage regulator is provided to the battery so that the electronic device stably operates without fluctuations of an output voltage to be supplied to the electronic device, even when a charge state of the battery fluctuates. The voltage regulator operates so as to stably operate the electronic device without the fluctuations of the output voltage supplied to the electronic device, even when a load applied by the electronic device rapidly fluctuates. Some voltage regulators include a control circuit for further stabilizing the output voltage of the voltage regulator.
Hereinafter, a description is given of a voltage regulator including the control circuit mounted thereto, which is proposed in JP 2005-352715 A. FIG. 3 is a circuit diagram of the voltage regulator of the related art.
An output voltage VOUT is divided by a resistor R13 and a resistor R14, with the result that a divided voltage VFB is obtained. An error amplifier circuit 31 compares the divided voltage VFB with a reference voltage VREF1, and operates so that the divided voltage VFB matches the reference voltage VREF1. Based on comparison results of the error amplifier circuit 31, a PMOS 32 is controlled, to thereby keep the output voltage VOUT constant.
In a case where the output voltage VOUT does not fluctuate transiently, a signal adding circuit 33 outputs a reference voltage VREF2 to an NMOS 31. A voltage between a gate and a source of the NMOS 31 does not exceed a threshold voltage of the NMOS 31, so the NMOS 31 does not operate. Accordingly, the control circuit 35 does not control the PMOS 32.
In a case where the output voltage VOUT drops transiently, a voltage at a predetermined internal node of the error amplifier circuit 31 rises transiently. The voltage at the internal node, which fluctuates transiently, is detected by a detection circuit 32. The signal adding circuit 33 adds the voltage, which is detected by the detection circuit 32, to the reference voltage VREF2, and outputs an addition result to the NMOS 31. The voltage between a gate and a source of the NMOS 31 exceeds the threshold voltage of the NMOS 31, so the NMOS 31 operates. Accordingly, the control circuit 35 controls the PMOS 32. Specifically, the NMOS 31 causes a current to flow, whereby a gate voltage of the PMOS 32 drops and the PMOS 32 is turned on. Then, the output voltage VOUT rises and the output voltage VOUT is kept constant.
Further, a description is given of a voltage regulator having a control circuit mounted thereto, which is proposed in Hoi Lee, K. T. Mok, Ka Nang Leung, “Design of Low-Power Analog Drivers Based on Slew-Rate Enhancement Circuits for CMOS Low-Dropout Regulators,” IEEE TRANSACTIONS ON CIRCUIT AND SYSTEMS. FIG. 4 is a circuit diagram of the voltage regulator of the related art.
In a case where an output voltage VOUT drops transiently, a voltage at a predetermined internal node of an error amplifier circuit 25 rises transiently. The voltage at the internal node, which fluctuates transiently, is detected by a control circuit 26. The control circuit 26 outputs a detection result to a PMOS 35. Then, a gate voltage of the PMOS 35 drops and the PMOS 35 is turned on. After that, the output voltage VOUT rises and the output voltage is kept constant.
Incidentally, the output voltage drops transiently not only due to the rapid fluctuations of the load connected to an output terminal, but also due to a protection function for stopping an output of the voltage regulator according to an overcurrent state and an overheat state of the voltage regulator.
When the load rapidly fluctuates, the control circuit 35 may perform an operation of detecting a reduction in the output voltage VOUT to increase the output voltage VOUT. However, if the control circuit 35 performs the above-mentioned operation when the protection function is activated, the output voltage VOUT rises despite the fact that the output of the voltage regulator is stopped so as to protect the voltage regulator. As a result, the protection function of the voltage regulator does not work. Accordingly, the safety of the voltage regulator is reduced.